To provide a composite floor slab which can be formed into minimum plate thickness required for strength of a base plate made of steel plate to reduce the weight, and eliminates degradation of fatigue strength of the steel base plate due to welding.

This composite floor slab 20 is constituted by forming a concrete layer 22 on a skin plate 21. On the upper face of the skin plate 21, Z-beams 30 in which each flanges 32, 33 is projected in the opposite directions on upper and lower both edge parts of a wainscot 31 to form into a Z-section, are fastened and fixed through the lower flanges 33 by high tension bolts and nuts 40, fixedly arranged in the direction of lateral axis at fixed intervals, and the Z-beams 30 are buried in the concrete layer 22 so that the skin plate 21 and the concrete layer 22 are combined together capably of being compounded in strength.